1. Field of the Invention
The present invention relates to a power output apparatus and a method of controlling the same. More specifically the present invention pertains to a power output apparatus for outputting power to a drive shaft and a method of controlling such a power output apparatus.
2. Description of the Prior Art
Proposed power output apparatuses include an internal combustion engine, a planetary gear as three shaft-type power input/output means, two motors (a first motor and a second motor) that can input and output powers, a battery supplying and receiving electrical energy required for inputting and outputting powers to and from the two motors, and a controller for controlling the internal combustion engine and the two motors (for example, Federal Republic of Germany Patent Application DE4124479A1). In this known power output apparatus, a crankshaft or an output shaft of the internal combustion engine is linked with a sun gear shaft, which connects with a sun gear of the planetary gear, via a one-way clutch, whereas a carrier shaft, which connects with a planetary carrier of the planetary gear, is linked with driving wheels via a differential gear. A rotor of the second motor is attached to the sun gear shaft, so that power is transmitted between the second motor and the sun gear shaft. A rotor of the first motor is attached to a ring gear shaft, which connects with a ring gear of the planetary gear, so that power is transmitted between the first motor and the ring gear shaft. The power output apparatus further includes a clutch that connects the sun gear shaft with the ring gear shaft and thereby integrates the three shafts (sun gear shaft, ring gear shaft, and carrier shaft) linked with the planetary gear.
In this proposed power output apparatus, the sum of the power output from the internal combustion engine and the power input to or output from the second motor is given to the planetary gear, while the first motor gives a reaction force to the ring gear. This enables the power to be output to the carrier shaft and thereby drives the driving wheels. In order to give a braking force to the driving wheels, the clutch is connected to integrate the three shafts the planetary gear and make the first motor and the second motor function as generators.
In case that the battery is in a fully charged state, however, the known power output apparatus can not make the first motor and the second motor function as generators nor output the braking force to the driving wheels. In this power output apparatus, the crankshaft and the sun gear shaft are connected to each other via the one-way clutch, so that a torque for rotating the internal combustion engine can not be transmitted from the sun gear shaft to the crankshaft. Namely the braking force can not be output to the driving wheels in the form of engine brake. Additional devices, such as a hydraulic circuit and an actuator, are required to ensure the operation of the clutch connecting the sun gear shaft to the ring gear shaft. This undesirably makes the whole power output apparatus complicated and bulky.